1. Field of the Invention
The present invention relates to a method for producing an organic electroluminescence (hereinbelow, abbreviated as the organic EL) display element.
2. Discussion of Background
Along with the rapid progress in the technical developments in the information communication field in recent years, more expectations are placed on flat display devices instead of CRTs. Among them, an organic EL display element is excellent in the high response, visibility, luminance, etc. and various studies are being made thereon.
The organic EL display element announced by Tang et al. of Eastman Kodak, U.S.A. in 1987 has a two-layered lamination structure of thin organic films wherein tris (8-quinolinolato) aluminum (hereinbelow, abbreviated as “Alq”) as represented by the below-mentioned formula (III) is used for a light emitting layer, by which a high luminance as 1000 cd/m2 was obtained by driving it with a low voltage of 10V or less. This organic EL display element was an element emitting a green color light having a luminous efficiency of 1.5 lm/W (see, for example, Non-patent Document 1).

With respect to the organic EL display element, however, it is known that the luminance per current density decreases as the emission time passes. Accordingly, it is important to suppress such a luminance decrease rate and to improve the emission luminance lifetime of the organic EL display element.
As a method for improving the emission luminance lifetime of the organic EL display element, studies have been made on a method of using a material having high heat resistance for the hole transport layer or a method of doping a dye for extending the lifetime in the light emitting layer.
Further, in attention to characteristics that the luminance decreasing in a continuous voltage application time is remarkable at the initial stage of the continuous voltage application time, an aging method capable of suppressing the luminance deterioration to obtain stable characteristics for a long term has been proposed.
As aging methods, a method of supplying a high density of current (see, for example, Patent Document 1), a method of applying a forward or reverse stair-like undulation voltage to the light emitting layer until it reaches the voltage value for driving under the condition that the potential of the anode is higher than the potential of the cathode (see, for example, Patent Document 2) and a method of stabilizing the organic EL display element by a heat treatment of 50° C. or higher (see, for example, Patent Document 3), have been proposed. However, even by such treatments, the effect of improving the emission luminance lifetime is still insufficient.
Patent Document 1: JP-A-8-185979 (paragraph No. 0009)
Patent Document 2: JP-A-4-14794 (scope of the claim)
Patent Document 3: JP-A-5-182764 (paragraph Nos. 0012–0014)
Patent Document 4: JP-A-2-311591 (scope of the claim)
Patent Document 6: JP-A-2-289675 (scope of the claim)
Patent Document 7: JP-A-2-216791 (scope of the claim)
Patent Document 8: JP-A-1-245087 (scope of the claim)
Patent Document 9: JP-A-3-33183 (scope of the claim)
Patent Document 10: JP-A-5-331459 (paragraph Nos. 0005–0014)
Patent Document 11: JP-A-7-90260 (paragraph No. 0019)
Non-patent Document 1: “Applied Physics Letter”, U.S.A., 1987, Vol. 51, p. 913
Non-patent Document 2: “Applied Physics Letter”, U.S.A., 1991, Vol. 59, p. 2760
Non-patent Document 3: “Applied Physics Letter”, U.S.A., 1997, Vol. 70, p. 152
It is an object of the present invention to provide a method for producing an organic EL display element for improving the emission luminance lifetime and stabilizing emission characteristics. It is another object and advantages of the present invention will be made clear from the following description.